1. Field of the Invention
The present invention relates to a connection structure in which electrodes that are made of indium-tin-oxide (ITO) or the like and are formed on an overcoat layer on a substrate are connected to other electrode terminals using an anisotropically electroconductive adhesive.
2. Description of the Related Art
As shown in FIG. 3, broadly speaking a liquid crystal panel 1 of a Super-Twisted Nematic (STN) color liquid crystal display or the like has a structure in which: a lower substrate 6xe2x80x94in which color filters 3 are formed on a glass substrate 2, an overcoat layer 4 is formed on top of these color filters, the surface is flattened, and lateral-stripe-like electrodes 5 made of ITO are formed on top of the overcoat layer 4xe2x80x94and an upper structure 9xe2x80x94in which longitudinal-stripe-like electrodes 8 made of ITO are formed on another glass substrate 7xe2x80x94are placed one on top of the other in such a way that the electrodes 5 and 8 of the lower and upper substrates form a lattice shape; the peripheral parts are sealed with a sealant 10; and liquid crystals 11 are held between the lower substrate 6 and the upper substrate 9.
The electrodes 5 of the liquid crystal panel 1 are connected to the electrode terminals 21 of the circuit board 20 of a tape carrier package (TCP) or the like by means of an anisotropically electro-conductive adhesive 30.
The anisotropically electroconductive adhesive 30 generally comprises electroconductive particles 32 dispersed in an insulating adhesive 31. In order to obtain highly reliable electrical continuity, a material of relatively high elasticity is used for the electroconductive particles 32. However, if electroconductive particles 32 of high elasticity are used with the ITO electrodes 5 on the overcoat layer 4, then cracks appear in the electrodes 5; a relatively soft material such as a styrene material or an acrylic material is thus used for the electroconductive particles 32.
However, even if a relatively soft material is used for the electroconductive particles 32, in the case that the overcoat layer 4 is thick (i.e. around 3 to 7 xcexcm), when the electrodes 5 on the overcoat layer 4 are connected to the electrode terminals 21 of the circuit board 20 by means of the anisotropically electroconductive adhesive 30, the electroconductive particles 32 encroach into the overcoat layer 4 as shown in FIG. 4, meaning that cracks still appear in the electrodes 5 on the overcoat layer, resulting in the problem of electrical connection becoming less reliable.
The present invention aims to resolve problems of conventional art such as the problem described above. Specifically, an object of the present invention is, in a connection structure in which electrodes on an overcoat layer are connected to other electrode terminals using an anisotropically electroconductive adhesive, to prevent cracks from appearing in the electrodes on the overcoat layer, thus improving the reliability of electrical connection, this being both when the electro-conductive particles in the anisotropically electro-conductive adhesive have a high elastic modulus and when the overcoat layer is thick.
The present invention has been accomplished when the inventors discovered that cracking of the electrodes on the overcoat layer can be preventedxe2x80x94and hence reliable electrical connection can be obtainedxe2x80x94by optimizing the angle between the surface of the overcoat layer and the surface of an electro-conductive particle, where this angle depends on the extent to which the electroconductive particles are pushed into the overcoat layer.
Specifically, the present invention provides a connection structure configured such that electrodes formed on an overcoat layer on a substrate are connected to other electrode terminals using an anisotropically electroconductive adhesive comprising electroconductive particles dispersed in an insulating adhesive, wherein the xe2x80x98angle of encroachmentxe2x80x99 of the electroconductive particles into the overcoat layer is at least 135xc2x0.
Here, the xe2x80x98angle of encroachmentxe2x80x99 of the electroconductive particles into the overcoat layer refers to the angle A shown in FIGS. 1A to 1F and FIGS. 2A to 2E, namely the angle between (a) the tangent to the surface of an electroconductive particle 32 and (b) the surface of the overcoat layer 4, at the point where the electroconductive particle 32 and the surface of the overcoat layer 4 intersect. Moreover, with the present invention, the angle of encroachment A is not actually measured by, for example, looking at the connection structure through a microscope, but rather is determined from the particle diameter of the electroconductive particles 32 and the thickness of the overcoat layer 4, assuming that in the connection structure the electroconductive particles are pushed in as far as the lower surface of the overcoat layer 4 (i.e. as far as the surface of the substrate 2).